There is increasing interest in phytoestrogens-plant compounds with structural or functional similarity to estradiol- as a possible alternative to estrogen replacement therapy. Soybeans and their products are the principal source of the isoflavones, genistein and daidzein. This study will test the hypothesis that soybean isoflavones increase calcium absorption and enhance calcium retention in postmenopausal women. The study design is a randomized, crossover, intervention study. Twenty postmenopausal women will be studied three times under different dietary interventions: soy protein enriched with isoflavones; soy protein devoid of isoflavones; and a casein control. Using a combination of metabolic balance methodology, kinetic tracer techniques, and biochemical and hormonal measures of bone metabolism, the following parameters will be measured: calcium absorption, calcium excretion, net calcium retention, bone formation, bone resorption, body calcium pool sizes and calcium turnover rates.